A Ni alloy, a Pt alloy, or an Ir alloy has been used as a spark plug electrode material (see Patent Document 1). A Ni alloy is inexpensive, but exhibits poor spark wear resistance. Therefore, a spark plug made of a Ni alloy must be frequently replaced. A Pt alloy and an Ir alloy, which are noble metal alloy, provide a spark plug with a long lifetime due to excellent oxidation resistance and excellent spark wear resistance, but are expensive. An electrode obtained by welding a chip made of a Pt alloy or an Ir alloy to the tip end of a Ni alloy electrode (i.e., only the tip end of the electrode is formed of a Pt alloy or an Ir alloy) has been proposed in order to reduce cost. However, peeling the chip off or the like may occur due to a difference in coefficient of linear expansion (see Patent Document 2). Thus, a spark plug electrode material that is inexpensive and has a sufficient lifetime has not been proposed.
In recent years, a spark plug electrode has been exposed to a severe environment along with an improvement in engine fuel efficiency, a high performance, and the like. Therefore, an electrode material that exhibits more excellent resistance (e.g., oxidation resistance and spark wear resistance) has been increasingly desired. Ti3SiC2 that exhibits excellent spark wear resistance, oxidation resistance, and heat resistance, and is cheaper than an alloy material has been proposed as an alternative to an alloy material (see Patent Document 3).
Ti3SiC2 exhibits high thermal conductivity, electrical conductivity, thermal shock resistance, and workability (properties of a metal) and excellent heat resistance and oxidation resistance (properties of a ceramic), and is considered to be a promising spark plug electrode material. However, since it is difficult to synthesize single-phase Ti3SiC2, TiC and SiC may be present in the resulting Ti3SiC2 as heterophases. Moreover, since it is difficult to sufficiently densify Ti3SiC2 under normal pressure, the resulting Ti3SiC2 may have an open porosity of several percents (see Patent Documents 4 to 6).